From the viewpoint of weight reduction for the purpose of increasing output power of engines and fuel economy, high fatigue strength is required for mechanical parts such as for automobiles, industrial machinery, construction machinery, and so on. Simply imparting high fatigue strength to steel can be easily achieved by increasing the hardness of steel by utilizing alloying elements and/or heat treatment. However, in general, the above described mechanical parts are formed by hot forging and thereafter finished into a predetermined product shape by cutting process. For this reason, the steel to be used as the starting material for the above described mechanical parts must have high fatigue strength and satisfactory machinability at the same time. In general, as the hardness of the starting material increases, the fatigue strength increases. On the other hand, regarding machinability, as the hardness of the starting material increases, cutting resistance and tool life tend to deteriorate.
Accordingly, to achieve fatigue strength and machinability at the same time, various techniques have been disclosed which allow the hardness to be suppressed to a low level in a forming stage in which high machinability is required and, on the other hand, allow the hardness to be increased by thereafter performing aging treatment in a final product stage in which strength is required.
For example, Patent Document 1 discloses the following age-hardening steel.
That is, there is disclosed an “age-hardening steel” containing: by mass %, C: 0.11 to 0.60%, Si: 0.03 to 3.0%, Mn: 0.01 to 2.5%, Mo: 0.3 to 4.0%, V: 0.05 to 0.5%, and Cr: 0.1 to 3.0%, and further containing, as needed, one or more kinds of Al: 0.001 to 0.3%, N: 0.005 to 0.025%, Nb: 0.5% or less, Ti: 0.5% or less, Zr: 0.5% or less, Cu: 1.0% or less, Ni: 1.0% or less, S: 0.01 to 0.20%, Ca: 0.003 to 0.010%, Pb: 0.3% or less and Bi: 0.3% or less, with the balance being Fe and inevitable impurities, wherein the following relationships are established among each component:4C+Mn+0.7Cr+0.6Mo−0.2V≥2.5,C≥Mo/16+V/5.7,V+0.15Mo≥0.4and wherein after rolling, forging, or solution treatment, the steel is cooled at an average cooling velocity of 0.05 to 10° C./sec in a temperature range of 800° C. to 300° C. so that before the aging treatment, an area fraction of bainite structure is not less than 50%, hardness thereof is not more than 40 HRC, and the hardness becomes 7 HRC or more higher than that before the aging treatment, due to the aging treatment.
Patent Document 2 discloses the following bainite steel.
That is, there is disclosed a “bainite steel”, containing: by mass %, C: 0.14 to 0.35%, Si: 0.05 to 0.70%, Mn: 1.10 to 2.30%, S: 0.003 to 0.120%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.40%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.30%, and V: 0.05 to 0.45% and further containing, as needed, one or more kinds selected from Ti: 0.001 to 0.100%, and Ca: 0.0003 to 0.0100%, with the balance being Fe and inevitable impurities, wherein the following relationships are satisfied:13[C]+8[Si]+10[Mn]+3[Cu]+3[Ni]+22[Mo]+11[V]≤30,5[C]+[Si]+2[Mn]+3[Cr]+2[Mo]+4[V]≤7.3,2.4≤0.3[C]+1.1[Mn]+0.2[Cu]+0.2[Ni]+1.2[Cr]+1.1[Mo]+0.2[V]≤3.1,2.5≤[C]+[Si]+4[Mo]+9[V], and[C]≥[Mo]/16+[V]/3.
Patent Document 3 discloses the following age-hardening type high-strength bainite steel.
That is, there is proposed an age-hardening type high-strength bainite steel having a chemical composition containing: by mass %, C: 0.06 to 0.20%, Si: 0.03 to 1.00%, Mn: 1.50 to 3.00%, Cr: 0.50 to 2.00%, Mo: 0.05 to 1.00%, Al: 0.002 to 0.100%, V: 0.51 to 1.00%, N: 0.0080 to 0.0200%, and further containing, as needed, one or more kinds selected from Ti: 0.01 to 0.10%, Nb: 0.01 to 0.10%, S: 0.04 to 0.12%, Pb: 0.01 to 0.30%, Ca: 0.0005 to 0.01%, and REM: 0.001 to 0.10%, with the balance being Fe and inevitable impurities, wherein
the steel is hot rolled or hot forged at a heating temperature of 1150 to 1300° C., and thereafter is cooled to a temperature not more than 200° C. with an average cooling velocity CV (° C./min) in a temperature range of 800 to 500° C. being kept as 40/(Mn %+0.8Cr %+1.2Mo %)≤CV≤500/(Mn %+0.8% Cr+1.2Mo %) such that hardness is not more than 400 HV, and micro-structure has a bainite ratio of not less than 70% and a crystal grain diameter of prior austenite of not more than 80 μm, and wherein
thereafter the steel is, as needed, subjected to cutting process or plastic working, and is farther subjected to aging treatment at a temperature of 550 to 700° C. such that a yield point or 0.2% yield stress is not less than 900 MPa.
Further, Patent Documents 4 and 5 disclose age-hardenable steels having a predetermined chemical composition or micro structure, and Patent Documents 6 and 7 disclose, as a method for obtaining steel parts for mechanical structures, a method of performing aging treatment, in which steel material is cooled at a predetermined cooling velocity after hot forging, and thereafter is subjected to aging treatment in a predetermined temperature range.